In the realm of spaceflight, manned and unmanned systems have been used to dock with and extend the life of, control, or dispose of other spacecraft, e.g., satellites. In this regard, an “Apollo Command/Service Module” docked with and modified the orbit of a Lunar Module, the “Atlas/Agena” unmanned booster docked with the “Gemini X,” “XI,” and “XII” and modified the coupled system's orbit, and “Progress” docked with, boosted, and later safely de-orbited the Russian Mir space station. In addition, Progress docks with and maintains the orbit of the National Aeronautics and Space Administration/European Space Agency/Russian (NASA/ESA/Russian) International Space Station (ISS).
Other systems have been designed to provide re-boost capabilities. For example, NASA designed an Orbital Maneuvering Vehicle in 1986 that was designed to re-boost the Hubble Space Telescope, the Gamma Ray Observatory, and other government and commercial payloads. Further, ESA is currently building the Automated Transfer Vehicle (ATV), to deliver supplies and perform re-boost for the ISS. Other efforts have recently been completed or are in progress for rendezvous and or docking of dissimilar spacecraft. The German space agency, Deutschen Zentrum für Luft- und Raumfahrt (DLR) and the National Space Development Agency of Japan (NASDA) performed an in orbit docking experiment in 1998 called ETS-VII (GETEX) which proved many concepts related to proximity operations of multiple spacecraft as well as robotic docking. They also addressed the system design issue of the momentum imparted to a combined spacecraft system by the use of robotic arms. The Defense Advanced Projects Agency (DARPA) is currently funding the Orbital Express mission as well as the “Demonstration of Autonomous Rendezvous Technology” (DART) rendezvous mission. However, none of these missions are targeted at spacecraft in geostationary orbit.
Typically, the life span of a geostationary Earth Orbit (GEO) spacecraft is upward of 15 years and is limited principally by the exhaustion of station keeping fuel. This fuel is needed in order to maintain the spacecraft's position over the earth's equator at an orbital altitude of approximately 35,800 kilometers. The orbital position of a GEO spacecraft is influenced primarily by the dissimilar gravitational forces exerted by the Earth, Moon, and Sun, hereinafter referred to collectively as “gravitational forces.” Such gravitational forces result in a drift of the spacecraft from the desired orbital position, hereinafter referred to as “spacecraft drift.” Spacecraft drift is unacceptable for the provision of services from these locations and it is typically desired that such drift be minimized.
Spacecraft drift comes in two forms, i.e., semi-major axis drift and inclination drift. Semi-major axis spacecraft drift results in an east/west drift from the desired orbital position and is hereinafter referred to as “east/west drift.” Inclination spacecraft drift results in spacecraft displacement that is orthogonal to the semi-major axis of the spacecraft orbit. Inclination spacecraft drift requires approximately ten times the energy to correct than semi-major axis spacecraft drift. Inclination drift is hereinafter referred to as “north/south drift.” Both east/west and north/south drift are typically corrected by a set of spacecraft thrusters.
In the parlance of the art, note that geosynchronous orbit refers to an orbit, whereby the orbital velocity of a spacecraft is equivalent to the rotational velocity of the Earth. A “geostationary orbit” (GEO) is a term that refers to zero degree inclination orbit around the Earth having a period of approximately 24 hours, i.e., a spacecraft in GEO orbit appears to hang motionless with respect to one's position on earth. Thus, a satellite in GEO orbit travels at a velocity equal to that of the rotation of the earth in order to remain in a relatively fixed position with respect to the earth. In order to remain in the equatorial plane (zero degree or Clarke Belt) as well as in a desired altitude (within 80 km) a propulsion system is typically employed.
It is relatively simple in energy terms to compensate for the east/west drift by firing thrusters along the velocity vector or toward nadir/zenith, because the east/west drift is a change in the eccentricity or period of the orbit. However, it is more difficult to compensate for the north/south drift described herein.
Note that the north/south and east/west drift with respect to the position of a geostationary spacecraft are completely separate from any attitude perturbations of the spacecraft that are due to gravity gradient torque, solar torque, or internal mechanical displacement torque due to the movement of components or fuel depletion in internal tanks. Such attitude displacements are typically compensated for by use of the aforedescribed propulsion system and thrusters and/or momentum management devices.